In current systems for wireless telecommunications, such as cellular telephony, the transmitters of the system, particularly those in the base stations, may be equipped with dual polarized antennas. Usually, this is in order to utilize the polarization of the radio channel for polarization reception diversity, as a complement to, or instead of, spatial diversity in the base station.
Transmission is normally carried out on one of the antennas of the device, i.e. in this example the base station, but polarization transmission diversity where both antennas are used, also exists. In some cases, both antennas in the base station are used for transmission, with half (or some other proportion) of the signals being transmitted via one antenna, and the rest via the other. One purpose of such a transmission arrangement is to reduce combining losses which may occur when radio channels are combined in the base station after power amplification.
Transmission diversity schemes such as the closed loop TX diversity in WCDMA can be seen as beam steering based on feedback when antennas with identical, or nearly identical, polarization are used. When antennas with orthogonal, or nearly orthogonal, polarization are used, closed loop TX diversity can instead be seen as a polarization adaptation. Instead of feedback information, uplink measurements can be used for adaptation of the downlink transmission.
For example, in the case of beam steering, using an array antenna with multiple closely spaced identical radiating elements, downlink beam forming can be controlled based on uplink direction-of-arrival estimates.